This invention relates to a method and apparatus for sealing between the nozzle banks of an axial flow steam turbine.
Various valve arrangements are used to control the admission of steam into steam turbines. In one arrangement, the bottom wall of the steam chest is formed with a plurality of steam passages, each leading to a bank of steam nozzles. A precision machined seat is mounted in each passage. A valve is vertically movable into and out of engagement with each seat to control the admission of steam from the steam chest through the passages to the nozzle banks.
As the demand load on the turbine increases, the valves are moved upwardly from their seats in a predetermined sequential order, under the control of a governor, by a valve lift bar which is mounted in the steam chest above the series of seats for movement toward and from the same. The lift bar is formed with an aperture vertically aligned with each valve seat. Each valve has a stem that extends upwardly through an aperture and is slidable in the aperture. The upper end portions of the valve stems are threaded to receive stop nuts. When the lift bar is moved upwardly, it engages the stop nuts on the valve stems, and moves the valves upwardly from their seats. The sequence in which the valves are moved from their seats and the extent of such movement and hence the opening sequence of the nozzle banks is determined by the position of the stop nut on each of the valve stems.
The lift bar is reciprocated vertically in the steam chest by a servomotor which is controlled by the governor, whereby the valves are opened and closed sequentially according to the load demand on the turbine. The sequential, rather than simultaneous, operation is employed to provide for the admission of steam to selected banks of nozzles in a predetermined order for more efficient turbine operation and for better balance with minimum vibration. Accordingly, upon initial upward movement of the lift bar, a valve having its stop nut adjusted close to the lift bar will be first moved to open position. The valve of the series having the stop nut on its stem adjusted to a higher position will open upon further upward movement of the lift bar, and so on. With the lift bar in its uppermost position, all of the valves are open and all of the nozzle banks are being fed, and, as the lift bar is moved downwardly, the valves and their associated nozzle banks are closed in reverse sequential order.
In high pressure steam turbine applications, a nozzle ring bolted to a multivalve steam end casing is highly undesirable. This is because bolts break due to the higher steam temperature and pressure and, additionally, require more radial space thereby limiting the maximum rotor shaft diameter required to maintain a desired critical speed. The ideal attachment is a rolled in nozzle ring secured in a groove machined in the casing to produce a tee-shaped tongue-and-groove relationship. However, to assemble this type of nozzle ring, a clearance must be provided. When the nozzle is assembled, this clearance is a leakage flow path which permits the steam to leak into the adjoining valve chambers and nozzle banks within the casing.